20个 javascript 技巧

1、变量的声明

//Longhand 
let x; 
let y = 20; 

//Shorthand 
let x, y = 20;

2、给多个变量赋值

通过数组解构，我们可以在一行中为多个变量赋值。

//Longhand 
let a, b, c; 
a = 5; 
b = 8; 
c = 12;
 
//Shorthand 
let [a, b, c] = [5, 8, 12];

3、三元运算符

使用三元(条件)运算符，我们可以节省5行代码。

//Longhand 
let marks = 26; 
let result; 
if(marks >= 30){
 result = 'Pass'; 
}else{ 
 result = 'Fail'; 
} 
//Shorthand 
let result = marks >= 30 ? 'Pass' : 'Fail';

4、指定默认值

//Longhand 
let imagePath; 
let path = getImagePath(); 
if(path !== null && path !== undefined && path !== '') { 
  imagePath = path; 
} else { 
  imagePath = 'default.jpg'; 
} 

//Shorthand 
let imagePath = getImagePath() || 'default.jpg';

5、&&

//Longhand 
if (isLoggedin) {
 goToHomepage(); 
} 

//Shorthand 
isLoggedin && goToHomepage();

6、交换两个变量

为了交换两个变量，我们通常使用第三个变量。通过数组解构赋值，可以很容易地交换两个变量。

let x = 'Hello', y = 55; 
//Longhand 
const temp = x; 
x = y; 
y = temp; 

//Shorthand 
[x, y] = [y, x];

7、箭头函数

//Longhand 
function add(num1, num2) { 
   return num1 + num2; 
} 

//Shorthand 
const add = (num1, num2) => num1 + num2;

8、模板文字

我们通常使用+操作符来连接字符串值和变量。使用ES6模板字面量，我们可以用一种更简单的方式来完成它。

//Longhand 
console.log('You got a missed call from ' + number + ' at ' + time); 

//Shorthand 
console.log(`You got a missed call from ${number} at ${time}`);

9、多行字符串

对于多行字符串，我们通常使用+操作符和新的换行序列(\n)。我们可以通过使用反撇号(')更简单地做到这一点。

//Longhand 
console.log('JavaScript, often abbreviated as JS, is a\n' +             'programming language that conforms to the \n' + 
'ECMAScript specification. JavaScript is high-level,\n' + 
'often just-in-time compiled, and multi-paradigm.' ); 
//Shorthand 
console.log(`JavaScript, often abbreviated as JS, is a programming language that conforms to the ECMAScript specification. JavaScript is high-level, often just-in-time compiled, and multi-paradigm.`);

10、多个条件检查

对于多值匹配，可以将所有值放在array中，并使用indexOf()或include()方法。

//Longhand 
if (value === 1 || value === 'one' || value === 2 || value === 'two') { 
     // Execute some code 
} 

// Shorthand 1
if ([1, 'one', 2, 'two'].indexOf(value) >= 0) { 
    // Execute some code 
}
// Shorthand 2
if ([1, 'one', 2, 'two'].includes(value)) { 
    // Execute some code 
}

11、es6的对象简写

let firstname = 'Amitav'; 
let lastname = 'Mishra'; 
//Longhand 
let obj = {firstname: firstname, lastname: lastname}; 

//Shorthand 
let obj = {firstname, lastname};

12、字符串转为数字类型

//Longhand 
let total = parseInt('453'); 
let average = parseFloat('42.6'); 

//Shorthand 
let total = +'453'; 
let average = +'42.6';

13、重复一个字符串多次

//Longhand 
let str = ''; 
for(let i = 0; i < 5; i ++) { 
  str += 'Hello '; 
} 
console.log(str); // Hello Hello Hello Hello Hello 

// Shorthand 
'Hello '.repeat(5);

14、指数的运算

//Longhand 
const power = Math.pow(4, 3); // 64 

// Shorthand 
const power = 4**3; // 64

15、~~ 是 math.floor（向下取整）的代替品

//Longhand 
const floor = Math.floor(6.8); // 6 

// Shorthand 
const floor = ~~6.8; // 6

16、找最大值

// Shorthand 
const arr = [2, 8, 15, 4]; 
Math.max(...arr); // 15 
Math.min(...arr); // 2

17、数组的遍历

除了for 的方法外，还可以用 for of ,for in ,当然还有es6 的遍历方法更方便

let arr = [10, 20, 30, 40]; 
//Longhand 
for (let i = 0; i < arr.length; i++) { 
  console.log(arr[i]); 
} 
//Shorthand 
//for of loop 
for (const val of arr) { 
  console.log(val); 
} 
//for in loop 
for (const index in arr) { 
  console.log(`index: ${index} and value: ${arr[index]}`); 
}

18、合并数组

let arr1 = [20, 30]; 
//Longhand 
let arr2 = arr1.concat([60, 80]); 
// [20, 30, 60, 80] 

//Shorthand 
let arr2 = [...arr1, 60, 80]; 
// [20, 30, 60, 80]

19、深拷贝

let obj = {x: 20, y: {z: 30}}; 

//Longhand 
const makeDeepClone = (obj) => { 
  let newObject = {}; 
  Object.keys(obj).map(key => { 
    if(typeof obj[key] === 'object'){ 
      newObject[key] = makeDeepClone(obj[key]); 
    } else { 
      newObject[key] = obj[key]; 
    } 
  }); 
 return newObject; 
} 
const cloneObj = makeDeepClone(obj); 

//Shorthand 
const cloneObj = JSON.parse(JSON.stringify(obj));
//Shorthand for single level object
let obj = {x: 20, y: 'hello'};
const cloneObj = {...obj};

20、从字符串中获取字符

let str = 'jscurious.com'; 
//Longhand 
str.charAt(2); // c 

//Shorthand 
str[2]; // c